I was told that the header pipes I could use are Mac's number 2591 or 2590.

I plan on picking one up this weekend. I'm going to try an APC 11x7, 12x6 and a 13x6. I probably will tune the pipe to 12x6 if, like the Evolution web site says that, the engine can turn 12,000 on the 12x6. The pipe should make that 13,000 to 13,500 or more I'd think. Stupid strong motor for a 4lb 11oz plane I plan to put it on!